1. Field of the Invention
The present invention relates to a ferrite sheet and, in particular, a ferrite sheet for use on a signal cable to prevent leakage and ingress of radio waves and/or to reduce or remove noise superimposed on a signal.
2. Description of the Related Art
Various wave absorbers for use on signal cables to prevent noise leakage from or ingress into electronic apparatuses or electronic components have been provided.
One known example of wave absorbers of this type is a wave absorber that includes a ferrite sintered body whose both surfaces are covered with tape, as described in Japanese Unexamined Patent Application Publication No. 2002-204094. As illustrated in FIG. 21, in the wave absorber, both surfaces of ferrite sintered bodies 63 are covered with pieces of tape 65 and 67. The wave absorber is attached to a signal cable 61 by bonding the tape 67 to the signal cable 61.
However, when the wave absorber described in Japanese Unexamined Patent Application Publication No. 2002-204094 is attached on the signal cable 61, the tape 67 is present between the signal cable 61 and the ferrite sintered bodies 63. Therefore, it is difficult for this wave absorber to sufficiently remove noise.
FIG. 22 is a graph illustrating frequency characteristics of impedance of a signal cable sandwiched between two ferrite sintered bodies, each having dimensions of 20.0 mm×15.0 mm×1.0 mm, for a distance (gap) between the ferrite sintered bodies of 0.2 mm, 0.3 mm, 0.4 mm, and 0.5 mm. As illustrated in FIG. 22, the mean value of impedance decreases with an increase in the gap. The mean value is reduced as much as approximately 10% from that for a gap of 0.3 mm to that for a gap of 0.4 mm, which is 0.1 mm larger than 0.3 mm. A reduction in the mean value of impedance results in a reduction in noise removal properties. This shows that the presence of the tape 67 between the signal cable 61 and the ferrite sintered bodies 63 increases the gap and thus results in the reduction in the noise prevention properties.
Depending on the material of the signal cable 61, bonding of the tape 67 and the signal cable 61 may be insufficient. This may cause the tape 67 to peel off by vibration of the signal cable 61, so there is a possibility to separate the ferrite sintered bodies 63 from the signal cable 61.